The invention relates to transducer amplifiers, and especially to transducer amplifiers that have at least as high common mode rejection as prior art transducer amplifiers, but have lower current drain and are less expensive.
Pressure transducers are commonly implemented by depositing strain sensitive resistance elements on a measurement diaphragm. Typically, the deposited strain sensitive resistors are connected in a Wheatstone bridge in such a fashion that one pair of resistors is put in compression and the other pair in tension as pressure is applied to the diaphragm, causing deformation thereof. Typically, two opposing corners of the bridge are connected to a DC voltage source, and the remaining corners produce the output signal of the Wheatstone bridge. The output signal includes a common mode voltage component produced by voltage division across the resistors when no pressure is applied to the transducer and an error signal component the amplitude of which is proportional to the resistance changes produced in the deposited resistors as the diaphragm is deformed due to application of pressure thereto. The common mode voltage component may be a number of volts, while the small signal or error component may be in the range from 1 to 20 millivolts, for permissible stress levels. Typically, the value of the common mode signal is not of interest, and only an amplified version of the error signal referenced to ground is desired. This presents the design engineer with the problem of how to amplify the error signals to produce the amplified error signal referenced to ground with adequate accuracy, minimum current drain and at minimum cost. These objectives can not be accomplished by using a simple differential input amplifier, as this approach will result in poor common mode rejection. Furthermore, such a circuit is unacceptably sensitive to thermal drift of the input offset voltage of the differential amplifier.
Therefore, instrumentation amplifiers, such as the one described in FIG. 6.5 of "Operational Amplifiers--Design and Applications" by Graeme, Huelsman, and Tobey, McGraw Hill, 1971 (incorporated herein by reference) have been used. (Those skilled in the art know that any amplifier that is connected to the output conductors of a transducer bridge has to have very low drift of its input offset voltage, so that large input offset errors will not be multiplied by the gain of that amplifier or subsequent amplifiers.) This "classical" instrumentation amplifier requires two precision, low drift operational amplifiers each having its positive input connected to one output terminal of the bridge, and each having a feedback resistor connected between its output and its negative input, and each having its negative inputs coupled together by a gain adjustment potentiometer. The differential output signal produced between two outputs of the operational amplifiers then is converted from a differential signal to a single ended output signal (i.e., a signal referenced to ground) by a typical differential to single ended output converter circuit of the type well known to those skilled in the art. Although the foregoing instrumentation amplifier circuit is widely used, it requires two costly high precision low drift operational amplifiers, rather than one, almost doubling the cost of this circuit over that required if only a single high precision, low drift operational amplifier could be used. Furthermore, such high cost, low drift operational or differential amplifiers all have relatively high current drain at the present state of the art, compared to cheaper, low current operational amplifiers that are readily available. High precision, low drift operational amplifiers typically have current drains of one to two milliamperes, whereas low cost low current operational amplifiers typically have only about ten to one hundred microamperes of current drain.
It is important to users of transducer amplifiers that they be able to conveniently change or "scale down" the calibration ranges of transducer amplifiers in accordance with large changes in the pressures or other parameters being measured by a transducer. For such scaling down to be accurate, it is essential that the drift of the input offset voltage be as low as possible.
It is also important that it be convenient to adjust a "zero point" of the output signal produced by a transducer amplifier to cause it to correspond to a desired point of the transducer output signal range. For classical instrumentation amplifiers, this has to be done by zero adjustment bias circuity connected either to the transducer bridge output, which causes undesireable bridge loading problems, or to the output of the transducer amplifier, which sometimes is difficult because of the available ground and power supply voltage values.
Thus, there remains an unmet need for a lower cost, lower current drain transducer output amplifier circuit that has performance at least as good, especially with respect to high common mode rejection, as the classical instrumentation amplifier that is almost universally used for this purpose, and also has as low input offset voltage drift as is practical, and also has its zero point easily adjusted.